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- 3.3.3 Sand Conclusions
- 3.4 Minimum Fluidising Velocity for Anchor Leg Model
- 3.5 Risk Assessment
- 3.6 Anchor Leg Modifications
Figure 11 Fluidised sharp sand
3.3.2 Void Fraction ComparisonA small amount of Kiln sand, silica of 0.5 mm, was purchased to compare the two void fractions. Void tests were carried out where it was measured how much water had to be added to roughly fill all voids for each type when the grains were dry. It was found that for 178 mL of dry silica sand, 79.76 mL of water was needed, giving a void fraction of 0.447. In comparison, 200 mL of dry sharp sand only needed 60.12 mL of water, resulting in void fraction of 0.301. From fluidisation investigation, it is known that the bigger void fraction was preferred. See to Appendix D for calculations. 3.3.3 Sand ConclusionsFigure 11 shows that sharp sand may be fluidised. There were however some concerns regarding the sand grain size. According to the contents data, the particles may be up to 4 mm, which is very large. There was a risk that the holes on the anchor leg may be blocked easily if the sand grains were this size. Also the fluidisation might not be as smooth, which was not desired when investigating the potential of the anchor leg as a concept. Kiln sand had more favourable void fraction. From these remarks, it became clear that the silica sand would be used for the experiments. 3.4 Minimum Fluidising Velocity for Anchor Leg ModelReferring back to section 2.2 about fluidised beds, the derived Ergun equation could be used to calculate the minimum fluidising velocity for the sand anchor. By assuming that Reynolds number is less than 1 and that the minimum void fraction is the same as void fraction found experimentally, the minimum fluidising velocity was able to be calculated. For rounded sand giving surface area - volume ratio of 0.83, this resulted in minimum fluidising velocity of 1.1 mm/s. If the ratio would be 0.6, then the minimum fluidising velocity nearly halved to 0.6 mm/s. See appendix E for detailed calculations. 3.5 Risk AssessmentThe following risks were identified for this particular project. Asbestos: This is located close to the top of the stairs where the experiments were carried out. Due to the potential danger of the protective layer being damaged, which would cause harm, instructions were carefully given by the asbestos supervising officer Gordon Duff to ensure safety. This included stressing the importance of using pig mats between the experiment and the asbestos so that any water spilt would not be able to reach this zone. Tripping hazard: Due to the experiment layout illustrated in section 3.1.1, there were many pieces of hoses as a result, where some were on the floor. Water butt failure: The water butt used is a standard design from B&Q, 0.21 m3 volume and made out of high-density polyethylene (HDPE) [BQ14]24. A water butt is designed to contain mainly water with potential leaves and dirt entering, but the experiment required a substantial quantity of sand along with the water to be held inside it. This mixture would have a very different density to water. Calculating the hoop stress and comparing it with the strength of HDPE material identified the safety factor of the water butt. For a thin-walled cylinder, a simplified water butt, the hoop stress is defined as: For this, the density of the sand and water mixture had to be calculated. It was assumed the sand would fill ¾ of the water butt. Its density when dry would be 1602 kg/m3 from [Eng14]26. The density of the wet silica sand had to be found since the gaps between the sand grains would create a different density depending on if they were filled with water or air. Very fine-grained sand, silica or silicon dioxide, has a density of 2600 kg/m3 if there is no space between grains [Kol09]27. The mass of air was assumed to be negligible compared to the mass of sand to find the volume occupied by the sand grains alone. Density of water from the tap was assumed to be 998 kg/m3, since the temperature would be roughly 15oC [Eng141]28. From this, two densities where calculated, one assuming the sand and water mixture was ¾ of the water butt; the other was for a worst-case scenario where the remaining quarter would have been filled with water. For the first case, hoop stress of 1.3 MPa was found giving a factor of safety of over 25; and for worst-case scenario the hoop stress would have been 1.5 MPa giving a factor of safety of roughly 22, since the tensile strength of HDPE is 32 MPa [Azo14]29. See Appendix H for full calculations. Air born sand particles: In section 3.3, choice of sand was investigated where in 3.3.1, the health issues regarding the sand particle size was evaluated. It was therefore known that the silica sand used should mainly be of particles that are too big to breath in. However, the few small grains would still have been a risk. Leaking water: Working with water travelling through various hoses would cause a risk of leakage. This could have been a problem if it came in contact with the asbestos or with electrical appliances. Electrical shock: If any electrical appliances were used during the experiments and they were in contact with water then electrical shock could have occurred. Falling: There are several stairs leading up to the top floor where experiments were carried out. Walking up and down several times during experiments to check on water flow rate at the bottom etc. increased risk of tripping. A risk assessment was carried out to highlight these potential hazards. The University of Edinburgh’s official risk assessment sheet, completed for this particular project, is found in Appendix F. It was however decided that more detailed risk assessment would be made, including the actions that needed to be taken both to prevent the risks and procedures to carry out if the hazards still occurred. This was organised in a table that is seen in Appendix G. The latter was agreed to be more useful for the experiments and was agreed on by Gordon Duff. 3.6 Anchor Leg ModificationsThe model anchor leg used for the experiments was a standard white 56 mm diameter plumbing pipe. Modifications then had to be made to create the model appropriate for testing. 3.6.1 Machining PartsWith a simple white draining pipe and standard garden watering supplies, some parts had to be machined to connect the anchor leg to the garden hose and also create an end fitting the white pipe. This was done using a lathe (see Figure 12). By machining PVC plastic and using o-rings, two secure and sealing ends were produced; where one was also threaded to fit a hose lock male adapter. These are seen in Figure 13 to Figure 15. 
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(3.1)[Eng142]25